Electrical insulating compositions based on olefin polymers

ABSTRACT

Electrical insulating materials based on olefin polymers and containing agents for increasing their electric strength. The electrical insulating materials according to the invention contain special azomethines as the agents for increasing electric strength. Insulations prepared from these materials do not fail even under prolonged influence of a high voltage.

United States Patent Wuerstlin et a1.

[ 1 July 4, 1972 [54] ELECTRICAL INSULATING COMPOSITIONS BASED ON OLEFINPOLYMERS [72] Inventors: Franz Wuerstlin; Franz Feichtmayr, both ofLudwigshafen, Germany [73] Assignee: Badische Anilin & Soda-FabrikAktiengesellschaft, Ludwigshafen/Rheinland, Rheinland-Pfalz, Germany[22] Filed: May 11, 1970 211 Appl. No.: 35,963

[30] Foreign Application Priority Data May 9, 1969 Germany ..P 19 23708.2

[52] US. Cl. ..252/63.2, 252/637, 260/45.9 R, 260/566 F [51] Int. Cl..C07c 119/00, C08f45/60, 1101b 3/18 [58] Field of Search ..252/63,63.2, 63.7; 260/566 F, 260/45.9 R

[56] References Cited UNITED STATES PATENTS 3,041,165 6/1962 Siis et al..96/1 3,303,162 2/1967 Fuchsman.....

3,321,436 5/1967 Stilz et a1. 260/45 8 3,350,312 10/1967 Gross et al...252/63.2 3,367,908 2/1968 Peterson et a1. ..260/45 .85

Primary ExaminerRobert F. Burnett Assistant Examiner-Robert A. DawsonAttorney-Johnston, Root, OKeeffe, Keil, Thompson & Shurtleff 57 ABSTRACTElectrical insulating materials based on olefin polymers and containingagents for increasing their electric strength. The electrical insulatingmaterials according to the invention contain special azomethines as theagents for increasing electric strength. Insulations prepared from thesematerials do not fail even under prolonged influence of a high voltage.

3 Claims, 1 Drawing Figure INVENTORS: FRA NZ WUERSTLIN FRANZ FEICHTMAYRATT'YS ELECTRICAL INSULATING COMPOSITIONS BASED ON OLEFIN POLYMERS Thepresent invention relates to electrical insulating compositions based onolefin polymers and containing agents for increasing electric strength.

Conventional electrical insulating materials (i.e., those without agentsfor increasing electrical strength) based on olefin polymers are knownto have a number of advantages over comparable other electricalinsulating materials. They have the disadvantage however that they haverelatively poor electric strength, i.e., that insulations preparedtherefrom fail under the influence of a relatively high electricpotential after only a short time or, in other words, have a short life.This property makes conventional electrical insulating compositionsbased on olefin polymers unsuitable for high-voltage applications, andparticularly unsuitable as insulating materials for high-tension cablesbecause in this application long life of the insulation under the actionof high electric field strengths is an essential prerequisite.

It is known from U.K. Patent Specifications Nos. 1,083,113 and l,083,l14 that this disadvantage can be overcome by adding to thepolyolefin-based electrical insulating materials special agents forincreasing their electric strength. These agents are chemical compoundswhich contain groups having a strong conjugation effect, particularlycompounds which contain one or more nitro groups with or without otherreactive groups. The use of such compounds is accompanied by a number ofother disadvantages: the compounds may be injurious to health and/orexplosive; they may give rise to undesirable secondary reactions ininsulating materials containing peroxides; or they may be solid with theconsequent risk of separation (efflorescence, formation of nodules).Since incorporation of the agent into the insulating composition and theprocessing of the insulating composition itself have to be carried outat fairly high temperatures (olefin polymers have fairly high meltingpoints) the first-mentioned disadvantages are particularly serious (riskof poisoning, risk of explosion, risk of decomposition of peroxide).

Other substances conventionally used for increasing the electricstrength of electrical insulating materials, namely unsaturatedperhalogenated hydrocarbon compounds (cf. German published applicationDAS 1,141,081), have the disadvantage that they tend to form hydrogenhalides. The special hydrocarbons proposed as agents for increasingelectric strength in German published application DAS 1,248,773 do nothave this disadvantage, but have to be used in undesirably largeamounts.

The present invention has for its object to provide electricalinsulating materials of the abovementioned type which do not have thesaid disadvantages or have them to a much lesser extent.

We have found that this object is achieved by using special azomethinesas agents for increasing electric strength.

The invention therefore consists in polyolefin-based electricalinsulating compositions which contain an agent for increasing electricstrength. Compositions in accordance with this invention contain, as theagent for increasing electric strength, a compound having the generalformula:

where p denotes one of the integers 0 to 4 and R' denotes a C, to Calkyl group or a C, to C alkoxy group, with the proviso that if pdenotes one of the integers 2 to 4 the radicals R may be identical ordifferent and the total number of carbon atoms in the radicals R doesnot exceed 18; and

R" denotes a C to C alkyl group or is identical with R.

Those compounds are preferred in whose formulae m denotes zero;

n denotes 1;

p denotes one of the integers O to 2;

R denotes methyl or ethyl;

R denotes a C or C alkyl group; and

R is identical with R.

In the preferred compounds it is advantageous for the groups X-R and--X-R" to be in metaor para-position to each other.

Specific examples of particularly suitable compounds are those havingthe following formulae:

(I) C 2H5 In every case it is advantageous if the azomethines are liquidat temperature below 50 C. It has further been found to be advantageousfor the azomethines to be used in an amount of from 0.5 to 5 percent byweight, preferably from 1 to 3 percent by weight, with reference to thetotal weight of the insulating composition. For the purposes of thepresent invention the azomethines may be used alone or as mixtures oftwo or more of these compounds.

The electrical insulating materials according to the invention are basedon olefin polymers. For the purposes of this invention, olefin polymersare defined as those conventionally understood by this term (cf. forexample the abovementioned UK. patent specifications). It includes inparticular homopolymers and copolymers of ethylene, especiallypolyethylenes having a density of from 0.918 to 0.960, preferably adensity of about 0.920. In addition to the olefin polymer, theinsulating compositions may contain conventional auxiliaries andadditives, for example crosslinking agents such as peroxides;stabilizers against thermal degradation; light stabilizers; carbon blackand other pigments or fillers.

' Production of the insulating materials from their components may becarried out in a conventional way with conventional equipment,particularly by homogenizing at temperatures of from 150 to 170 C inscrew or roller mixing equipment.

' Electrical insulating compositions according to this invention areparticularly suitable for cables and especially for hightension cables.

The invention is illustrated by the following Examples. Parts andpercentages specified in the Examples are by weight.

The measured value referred to as number of breakdowns" in the Examplesdetermined as follows: (.irculur tent discs are prepared in a press by aconventional method from the insulating material; they have a diameterof 69 mm and a thickness of 5 mm and are provided with a central conicaldepression (depth of cone 4 mm, vertex angle of the cone 50). Theconical depression and a part of the fiat surface on the reverse side(on a circle having a diameter of 25 mm) are metallized with silver(about 0.1 mm in thickness) (cf. the drawing, reference A).- The twometal layers serve as electrodes in the electric strength test, highvoltage being applied to the conical electrode by a needle, whereas theflat electrode on the reverse side is earthed. All tests are carried outbeneath the surface of oil.

An effective voltage of kilovolts (50 cycles per second) is applied to10 test discs of each type for 1 hour and the number of test discs whichhave failed is indicated in the appropriate column in the Table of theExamples. The remaining test discs (i.e., those which have not failed)are then subjected to an effective voltage of kilovolts (50 cycles persecond) for one hour and again the number of breakdowns is noted. Testdiscs which still have not failed are subjected to an effective load ofkilovolts (50 cycles per second) for 1 hour and again the number ofbreakdowns is registered. ln Example 1 the series is continued at aneffective load of 35, then and then kilovolts cycles per second), ineach case for 1 hour. It may be seen for example from the Table inExample 1 that polyethylene stabilized according to this inventionwithstands the first three voltage stages without breakdown and only inthe fourth stage two of the total of 10 test discs 20 percent), in thefifth stage five 50 percent) and in the sixth stage two 20 percent) ofthe discs fail. On the other hand, the same polyethylene which does nothowever contain an agent according to this invention fails to the extentof 80 percent (eight test discs) in the first voltage stage and to theextent of 20 percent (two test discs) in the next stage.

EXAMPLE 1 Number of breakdowns at 20 kv 25 kv 30 kv 35 kv 4O kv 45 kvExample I 0 0 0 2 5 2 Blank test 8 2 EXAMPLES 2 to 8 The procedurefollowed is completely analogous to that in 5 Example 1 but the typesand amount of azomethines indicated in the following Table are used. Thevalues obtained are also given in the Table. The Roman numeralsindicated in the Table refer to the azomethines with the formula givenunder these numerals in the description.

1. An electrical insulating composition consisting essentially of apolyethylene having a density of 0.91 8 to 0.960 and 0.5 to 5 percent byweight, based on said composition, of an agent for increasing electricalstrength selected from the group consisting of compounds of the formulaeI C H;

2. A composition as claimed in claim 1 wherein the amount of said agentis l to 3 percent by weight, based on said com-

2. A composition as claimed in claim 1 wherein the amount of said agentis 1 to 3 percent by weight, based on said composition.
 3. A compositionas claimed in claim 1, said polyethylene having a density of about0.920.